1. Field of the Invention
The present invention relates to a mobile communication system. More particularly, the present invention relates to a method for reducing the amount of feedback that a Mobile Station (MS) transmits to a Base Station (BS).
2. Description of the Related Art
The provision of services with diverse Quality of Service (QoS) requirements at high data rates is an area being studied for a future-generation communication system called a 4th Generation (4G) communication system. In particular, research is being conducted on providing a high-speed service that ensures mobility and QoS in a Broadband Wireless Access (BWA) communication system, such as Wireless Local Area Network (WLAN) and Wireless Metropolitan Area Network (WMAN).
For the 4 G communication system, the use of Orthogonal Frequency Division Multiplexing (OFDM)/Orthogonal Frequency Division Multiple Access (OFDMA) is being considered for high-speed data transmission on wired/wireless channels. OFDM/OFDMA is a data transmission scheme that uses multiple carriers.
An example of an OFDM/OFDMA system is an Institute of Electrical and Electronics Engineers (IEEE) 802.16 communication system. The IEEE 802.16 communication system is capable of high-speed data transmission because of its use of a plurality of subcarriers.
To support high-speed data transmission, the IEEE 802.16 communication system employs many techniques including Adaptive Modulation and Coding (AMC). AMC is a data transmission scheme that adaptively uses a modulation and coding scheme according to the channel state between the MS and a BS, thus improving the overall use efficiency of a cell.
For AMC, a plurality of modulation schemes and a plurality of coding schemes are defined and a channel signal is encoded and modulated using a combination of modulation and coding schemes. Combinations of the modulation and coding schemes are called Modulation and Coding Schemes (MCSs). According to the number of MCSs, level 1 to level N are defined. That is, AMC adaptively selects an MCS level for the MS according to the channel state between the MS and the BS. Therefore, the overall system efficiency is improved.
The use of AMC requires a feedback of information about the state of a downlink channel, i.e. the communication of a Channel Quality Indicator (CQI) from the MS to the BS in the IEEE 802.16 communication system.
Conventional feedback methods are described below.
(1) Best-M feedback: the MS feeds back the CQIs of M subchannels to the BS in a descending order of channel state.
FIG. 1 illustrates the conventional Best-M feedback method.
Referring to FIG. 1, each MS feeds back channel information about M subchannels. The Best-M feedback method works well in conjunction with Proportional Fairness (PF) scheduling in which the same number of resources, i.e. time slots, on average are allocated to each MS over a long term.
Max Carrier-to-Interference (C/I) scheduling maximizes the sum of the average data rates of MSs. Since an MS experiencing a relatively good channel state also feeds back information about M subchannels in the Best-M feedback method, the MS loses the chance of being allocated more subchannels. In terms of system capacity, the Best-M feedback method is not efficient when used in conjunction with Max C/I scheduling.
On the other hand, despite a low probability of being scheduled by a Max C/I scheduler, MSs experiencing a bad channel state feed back channel information about M subchannels, resulting in wasted resources. A Max-Min Fair scheduler allocates more time slots to the MSs experiencing the poor channel state to keep their data rates at the same level as those of MSs experiencing the good channel state. Consequently, as with Max C/I scheduling, Max-Min Fair scheduling causes a loss in fairness as well as feedback inefficiency when compared to the Best-M feedback method.
(2) Absolute Signal-to-Noise Ratio (SNR) Thresholding (AST) feedback: each MS performs a CQI feedback only when its SNR is equal to or higher than a preset threshold. The feedback condition is given byrk(t)≧rth  (1)where rk denotes the SNR of MS k and rth denotes the SNR threshold. According to equation (1), the MS determines whether to feed back an instantaneous SNR at time t.
Because MSs with SNRs below the threshold do not feed back their channel state information, the ATS feedback method excludes these MSs from scheduling, which is not favorable in terms of fairness.
(3) Normalized SNR Thresholding (NST) feedback: each MS feeds back channel information only if its normalized SNR is equal to or higher than a preset threshold. The feedback condition is
                                                        γ              k                        ⁡                          (              t              )                                            γ            k                          ≥        A                            (        2        )            where γk denotes the average SNR of MS k and A denotes the threshold. The NST feedback method is a case of the AST feedback method taking into account the characteristics of a PF scheduler. The NST feedback method is similar to the Best-M feedback method in that each MS feeds back channel information about a preset ratio of subchannels in good states to subchannels available to the MS over a long period of time. This means that NST feedback has the shortcomings of Best-M feedback.
An IEEE 802.16 communication system using the AMC feedback method for slow moving MSs can operate a differential feedback scheme along with the Best-M feedback method. Specifically, a slow moving MS transmits to a BS an REP-RSP message including a bitmap that indicates five bands and their CQIs during an initial feedback. Then the slow moving MS transmits the CQI differentials of the five bands in a 1-dB stepwise manner to the BS. If any of the five bands is changed, the slow moving MS transmits an REP-RSP message with a bitmap indicating the new five bands and their CQIs to the BS. Meanwhile, a fast moving MS can operate in diversity mode where it transmits one representative CQI being the average CQI of all of the bands.
If the channel variation of the slow or fast moving MS is larger than 1 dB, accurate channel tracking is not possible, which may cause prediction errors. Also, when the five bands change frequently, the resulting message exchanges may increase signaling overhead.